The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a low coercivity (free) ferromagnetic layer, a non-magnetic spacer layer, and a high coercivity ferromagnetic layer. The latter is usually formed out of a soft ferromagnetic layer that is pinned magnetically by a nearby layer of antiferromagnetic material. Alternatively, a synthetic antiferromagnet (formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers) may be used as the pinned layer. This results in a more stable device which we will refer to it as a synthetically pinned device.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 10–20% when current flow is in the film plane.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of both the leads and the other GMR stack layers should be as low as possible.
As shown in FIG. 1, the CPP spin valve structure has three magnetic layers: free layer 17 as well as AP1 layer 15, and AP2 layer 13. Free layer 17 is free to rotate in response to external fields. The AP2 direction is fixed by antiferromagnetic layer 12 (typically MnPt) with ruthenium layer 14 being used to provide the antiferromagnetic coupling. Their relative magnetization directions during device operation are always antiparallel to one other. It is normal practice to utilize the same material (like CoFe) for both AP1 and AP2. This has a positive bulk spin asymmetry coefficient β, as well as positive interface spin asymmetry coefficient γ.
β is defined as 1−ρ↑/(2ρ)=ρ↓/(2ρ)−1 where ρ↑, ρ↓ are the resistivity of spin up and spin down electrons, respectively. ρ is the material resistivity (=ρ↑ρ↓/ρ↑+ρ↓). γ is defined as 1−r↑/2rb)=r↓/(r↑+r↓) where r↑(r↓) is the interface resistance for spin up and spin down electrons; rb=(r↑r↓)/r↑+r↓). When r↑=r↓, γ will be 0 and the interface has no spin dependent scattering. Also seen in FIG. 1 is seed layer 11, capping layer 18 and non-magnetic spacer layer 16.
In TABLE I we show the β and γ magnitudes for the three magnetic layers together with the resulting magnitude of their resistivity for both up and down electrons for both the parallel and antiparallel states:
TABLE I(Ru between AP1 and AP2)resistivity in P stateresistivity in AP stateLAYERβγspin upspin downspin upspin downCoFe (free)>0>0lowhighhighlowCoFe (AP1)>0>0lowhighlowhighCoFe (AP2)>0>0highlowhighlow
The consequences of this are that the AP2 contribution to CPP GMR is always negative so it reduces the resistance contrast between the parallel and anti-parallel states of the free layer. This limits the GMR ratio as well as dRA (change in anti-parallel resistance) for synthetically pinned spin valves.
In order to meet higher signal requirements it would be desirable to reduce the thickness of the free layer besides improving the GMR ratio itself. However, thinning of the free layer causes a low GMR ratio and poor thermal stability. A synthetic free layer would seem to provide a way to maintain good thermal stability but, in both in CIP and CPP SV structures, synthetic free layers actually cause a GMR loss due to current shunting in CIP and effective thinning of the free layer in CPP.
The present invention discloses a solution to this problem.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,627,704, Lederman et al. show a MR CCP transducer structure. Dykes et al. (U.S. Pat. No. 5,668,688) shows a CPP SV MR device. U.S. Pat. No. 6,134,089 (Barr et al.) also describes a CPP MR device. U.S. Pat. No. 5,883,763 (Yuan) discloses a CPP GMR Transducer while in U.S. Pat. No. 5,657,191 Yaun teaches how to stabilize a MR device. U.S. Pat. No. 6,002,553 (Stearns et al.) and U.S. Pat. No. 5,446,613 (Rottmayer) also are related patents.